Nuclear power facilities, such as nuclear power plants and the like, are conventionally equipped with a filter for removing radioactive iodine. A flow of radioactive iodine-containing steam generated in a nuclear power facility is passed through the filter so that radioactive iodine is adsorbed and removed before being discharged from the nuclear power facility. This process is very important, and therefore, research and development efforts have been and are still being undertaken in order to improve the radioactive iodine adsorbing effect of the filter. Among such filters that have been developed are some radioactive iodine adsorbents made of zeolite. An example of those radioactive iodine adsorbents is one in which silver is supported on a zeolite having a silica-to-alumina molar ratio of 15 or more (see, for example, Patent Document 1). Patent Document 1 discloses such a radioactive iodine adsorbent that contains a reduced amount of silver supported on zeolite and can remove radioactive iodine with improved efficiency.